function add(c, d) {
  return this.a + this.b + c + d;
}
var o = { a: 1, b: 3 };
console.log(add.call(o, 5, 7)) // 1 + 3 + 5 + 7 = 16  this指向o 
console.log(add.apply(o, [10, 20])) // 1 + 3 + 10 + 20 = 34  this指向o 